c-axis infrared properties of Sm2-xCexCuO4-delta

We have measured the grazing-incidence p-polarized far-infrared reflectivity of Sm2-xCexCuO4-delta crystals (T-c = 15 K). C-axis longitudinal-optical phonon features are clearly visible at 430, 570, and 600 cm(-1). The phonon frequencies do not shift with temperature, which indicates that the electronic component of the dielectric function remains almost unaltered above the energy scale of 3-4k(B)T(c) across the superconducting transition. Below T-c the absorptivity (measured down to 30 cm(-1)) does not show a signature of a threshold at around 3.5k(B)T(c) characteristic of BCS superconductivity. Based on our observations with (E) over right arrow parallel to (c) over right arrow an upper limit of 30 cm(-1) can be placed on the energy of the screened Josephson plasmon for temperatures down to 4 K.